4. Results and discussion
4.2. Effect of paper types
37
Also, the thickness of graphene layers increases in proportion to the WDG contents, as verified Figure 4.2. The thermal conductivities are also investigated according to the coating thickness which is an important factor of film-typed TIM [35]. Regardless of the type of paper, it can be confirmed that the thermal conductivities increase as the coating thickness both in the in-plane and through-plane thermal conductivity. In the case of Munken, the in-plane thermal conductivity varies from 6.05 W/m- K to 9.24 W/m-K and the through-plane thermal conductivity varies from 0.1 W/m-K to 0.14 W/m-K depending on the thickness variation from 0.13 mm to 0.25 mm. For the Matt, as the thickness varies from 0.13 mm to 0.25 mm, the in-plane and through-plane thermal conductivity improves from 5.89 W/m-K to 10.11 W/m-K and 0.12 W/m-K to 0.26 W/m-K for the variation of thickness respectively. As with the mass fraction, this is due to an increase in the amount of WDG paste affected the thermal conductivity. This figure also shows that the in-plane thermal conductivity also linearly increases with the measured coating thickness. Furthermore, the increase of the in-plane thermal conductivity is much greater than the tendency for the through-plane thermal conductivity. The through-plane thermal conductivity is shown to be not significantly affected by the WDG content or coating thickness. The anisotropic thermal conductivities of graphene paper TIM considered in this study are due to the crystal structure of graphene which acts as a conductive fillers. Therefore, it is necessary to select an appropriate TIMs haivng high thermal conductivities as satisfying the conditions of the thickness for thermal management application.
38
Table 4.2. shows the effect of the paper types and thickness on both in-plane and through-plane thermal conductivities. It can be confirmed that the thermal conductivities are different according to the paper types, despite the same coating thickness of 3-mm thick [38]. Oriental traditional papers, Bulgyeong and Daerye, and wool paper show relatively high thermal conductivities. As in the previous results, it also shows that the in-plane thermal conductivities are relatively higher than the through- plane thermal conductivities.
In this regard, the porosity of each paper was investigated. Porosity means a measure of void spaces in a material to calculate the volume of void-space divided by total volume of material. The porosity of the paper except for the surface treated merit and aqua satin can be obtained by the following equation.
V T
V
=V
Where, ε, Vv, and VT represesnts porosity, volume of void-space, and total voume of material, respectively.
If the void space is filled with air, the porosity of papers is as
Bulk Particle
= −1
Where,
ρ
Bulk andρ
Particel means the density of paper and cellulose, respectively.39
Table 4.3. Basis weight of six-types of paper
The density of cellulose is 1.5 g/m3 and the basis weight which means g/m2 are as table 4.3.
Also, the density of paper is obtained basis weight divided by the thickness of each paper. To investigate porosity of surface treated merit and aqua satin, the additives were investigated. In the coated paper, the glossy paper has a pigment less than ~ 20 g/m2 and is typically art paper. Also, nonglossy paper means paper with pigment of less than ~ 10 g/m2, aqua satin and matt. Lightweight coated paper is less than ~ 5 g/m2 of pigment of merit. In this regard, the porosity of merit and aqua satin were obtained by considering the amount of pigment contained in the papers. The pigments used for surface coating of papers consist of clay and ground calcium carbonate (GCC, hereafter). Most of these pigments have a mass fraction of 30 to 70 of clay and GCC. Also, the contained volume of each element is obtained by considering properties of cellulose, clay, and GCC. The contained volume ratio of clay and GCC are less than 3 % in paper. As a result, the porosity of Merit and Aqua satin considering all of the cellulose, clay, and CGG can be obtained as 31.42 % and 41.56 %, respectively.
Papers Basis weight [g/m2]
Bulgyeong 85
Daerye 40
Merit 90
Aqua satin 128
Wool paper 128
New craft board 161
40
Bulgyeong Daerye Munken Wool Matt Craft board Merit Aqua satin 0.30
0.35 0.40 0.45 0.50 0.55 0.60 0.65
Types of paper
Porosity
4.0 4.5 5.0 5.5 6.0 6.5 7.0
Thermal conductivity [W/m-K]
Figure 4.3. Effect of paper types on the thermal conductivities
Figure 4.3 shows the effect of paper types on the in-plane thermal conductivities. The properties of each cellulose are same and additives are negligible other than pigments. As a result, it can be confirmed that the porosity which is characteristics of paper and thermal conductivities are somewhat proportional. It can be assumed that that the paper with high porosity can absorb WDG well and it achieves the enhanced heat transfer path with cellulose. However, most high porosity paper has a disadvantage of low mechanical strength, so it is necessary to select the appropriate paper suitable for the application.
From the measured thermal conductivities, it can be confirmed that in-plane thermal conductivities are significantly remarkable than the through-plane thermal conductivities. This is due to the properties of graphene which plays a role of thermally conductive filler of graphene paper TIMs.
41
Figure 4.4. Illustration of the crystal structure of graphene
Graphene is one of the carbon allotropes and the crystal structure is in-plane direction of a two-dimensional structure connected to each other in a hexagonal structure with alternating double bonds between carbon atoms as shown in the figure 4.4. [50].
(a)
(b)
Figure 4.5. Comparison of heat transfer path of (a) in-plane and (b) through-plane heat flow
42
In general, thermal conduction occurs when atomic vibrational energy is transmitted. In this regard, graphene has a strong bond extending between the carbon in the plane direction and can transmit the vibration energy more quickly for in-plane direction as shown in the figure 4.5.. Practically used graphene is present in laminated form but the van der Waals forces between the graphene atoms in the vertical direction are weak and the through-plane thermal conductivity is relatively low. In this way, due to the crystal structure of graphene which acts as a conductive filler, highly anisotropic thermal conductivity occurs. The highly anisotropic thermal conductivity of these cellulose-based graphene TIM is useful for thermal management in the form of a thin film for the latest electronic components.
However, it should be noted that the limitation through-plane thermal conductivity may be a bottleneck for useful thermal management systems.